Prefixes Set 1: in-, un-, re-

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Across
  1. 1. not formal
  2. 3. not seen
  3. 5. not valid
  4. 6. to turn back
  5. 8. to join again
  6. 10. to open a lock
  7. 11. brave
  8. 13. to call back
Down
  1. 1. not active
  2. 2. to build again
  3. 4. wrong
  4. 7. sad
  5. 9. dangerous
  6. 10. to put back the way it was
  7. 12. to appear again